The welded structures forming building structures naturally are required to be superior in the properties of the welded joints, but in recent years the property of being superior in tensile strength at a high temperature as so-called “fire-resistant steel” has further become demanded. This is due to the allowance of “performance-based designs” as a result of the findings of the “Development of Refractory Design Methods” studied in a technical development project of the Ministry of Construction (then) entitled “Development of Fireproof Design Methods for Buildings” promoted in the five years from fiscal 1982 to fiscal 1986. Due to this, it has become possible to determine what extent of fire-resistant covering is required by the high temperature strength of the steel material and the load actually applied to the building and it has become possible to use nonfire-resistant covering steel materials in accordance with the high temperature strength properties of steel materials (see General Fireproof Design Method of Buildings (Vol. 4)—Refractory Design Methods, the Building Center of Japan, Apr. 10, 1989).
Here, “fire-resistant performance” is performance of a steel material exhibiting the necessary strength for a certain time when the steel material is exposed to a fire in the state without a fire-resistant covering. This is so as to keep the building structure from collapsing and to thereby facilitate the escape of people residing in it. Various sizes of fires and ambient temperatures may be envisioned, so when not providing a fire-resistant covering on a steel material, the steel material supporting the strength of the structure is required to be as high in strength at a high temperature as possible.
In the past, R&D has been conducted on steel materials provided with such a fire-resistant performance. For example, steel materials improved in high temperature strength by the addition of a suitable quantity of Mo are being proposed (see Japanese Patent Publication (A) No. 2001-294984, Japanese Patent Publication (A) No. 10-096024, and Japanese Patent Publication (A) No. 2002-115022). These steel materials all envision use at less than 700° C. The high temperature strength is raised by precipitation strengthening of Mo carbides or by joint use of precipitation strengthening of other carbides and structural strengthening.
On the other hand, due to the pinch in the supply and demand of various types of alloy elements, the addition of Mo is raising the costs of steel materials industrially. Due to this, technology employing alloy designs involving other than the addition of Mo is being disclosed (see, for example, Japanese Patent Publication (A) No. 07-286233 and Japanese Patent No. 3635208). In the low yield ratio steel material for building use described in Japanese Patent Publication (A) No. 07-286233, B is added to improve the hardenability so as to secure high temperature strength at about 600° C. Further, in the low yield ratio fire-resistant steel plate described in Japanese Patent No. 3635208, the high temperature strength is improved by adding Cu, Mn, or another γ-phase stabilizing element.
Further, Japanese Patent Publication (A) No. 2006-249467 discloses a steel material superior in HAZ toughness of the welded joints improved in high temperature strength at 750° C. by compositely adding B and Mo.